Conventional heat transfer fluids such as water, mineral oil, and ethylene glycol play an important role in many industries including power generation, chemical production, air conditioning, transportation, and microelectronics. However, their inherently low thermal conductivities have hampered the development of energy-efficient heat transfer fluids that are required in a plethora of heat transfer applications. It has been demonstrated recently that the heat transfer properties of these conventional fluids can be significantly enhanced by dispersing nanometer-sized solid particles and fibers (i.e. nanoparticles) in fluids (Eastman, et al., Appl. Phys. Lett. (2001), 78(6):718-720; Choi, et al., Appl. Phys. Lett. (2001), 79(14):2252-2254). This new type of heat transfer suspension is referred to herein as a nanofluid. In particular, carbon nanotube-containing nanofluids provide several advantages over conventional fluids, including thermal conductivities far above those of traditional solid/liquid suspensions, a nonlinear relationship between thermal conductivity and concentration, strongly temperature-dependent thermal conductivity, and a significant increase in critical heat flux. Each of these features is highly desirable for thermal systems and together, they make nanofluids good candidates for the next generation of heat transfer fluids.
The observed substantial increases in the thermal conductivities of nanofluids can have broad industrial applications and can also potentially generate numerous economical and environmental benefits. Enhancement in the heat transfer ability could translate into high energy efficiency, better performance, and low operating costs. The need for maintenance and repair can also be minimized by developing a nanofluid with a better wear and load-carrying capacity. Consequently, classical heat dissipating systems widely used today can become smaller and lighter, thus resulting in better fuel efficiency, less emission, and a cleaner environment.
Nanoparticles of various materials have been used individually to make heat transfer fluids, including copper, aluminum, copper oxide, alumina, titania, and carbon nanotubes (Keblinski, et al, Material Today, (2005), 36-44). Of these nanoparticles, carbon nanotubes show the greatest promise due to their excellent chemical stability and extraordinary thermal conductivity. Carbon nanotubes are macromolecules having the shape of a long thin cylinder and thus have a high aspect ratio. There are two main types of carbon nanotubes: single-walled nanotubes (“SWNT”) and multi-walled nanotubes (“MWNT”). The structure of a single-walled carbon nanotube can be described as a single graphene sheet rolled into a seamless cylinder with ends that are either open, or capped by either half fullerenes or more complex structures such as pentagons. Multi-walled carbon nanotubes contains two or more nanotubes that are concentrically nested, like rings of a tree trunk, with a typical distance of approximately 0.34 nm between layers.
Carbon nanotubes are one of the most thermally conductive materials known today. Basic research over the past decade has shown that carbon nanotubes have a thermal conductivity an order of magnitude higher than copper (3,000 watts per meter Kevin (W/mK) for multi-walled carbon nanotubes and 6,000 W/mK for single-walled carbon nanotubes). Therefore, the thermal conductivities of nanofluids containing carbon nanutubes are significantly enhanced when compared with conventional fluids. A 150% increase in conductivity of oil that contains about 1% by volume of multi-walled carbon nanotubes has been reported recently (Choi, et al., App. Phys. Lett., (2001), 79(14):2252-2254).
Several additional studies of carbon nanotube suspensions in various heat transfer fluids have also been reported. However, only moderate enhancements in thermal conductivity have been observed. Xie, et al., reported that a carbon nanotube suspension in an aqueous solution of organic liquids results in only 10-20% increases in thermal conductivity at 1% by volume of carbon nanotubes (Xie, et al., J. Appl. Phys., (2003), 94(8):4967-4971). Similarly, Wen and Ding found an about 25% enhancement in the conductivity at about 0.8% by volume of carbon nanotubes in water (Wen and Ding, J. Thermophys. Heat Trans., (2004), 18:481-483). Even at these levels, carbon nanotubes still hold great promise as being the next generation of efficient thermal transfer fluids.
Despite the extraordinarily promising thermal properties exhibited by carbon nanotube suspensions, it remains to be a serious technical challenge to effectively and efficiently disperse carbon nanotubes into aqueous or organic media to produce a nanoparticle suspension with a sustainable stability and having consistent thermal properties. Due to the hydrophobic nature of graphitic structure, unmodified carbon nanotubes are not soluble in any known solvent. They also have a very high tendency to form aggregates and extended structures of linked nanoparticles, thus leading to phase separation, poor dispersion within a matrix, and poor adhesion to the host. However, stability of the nanoparticle suspension is essential for practical industrial applications. Otherwise, the thermal properties of a nanofluid, such as thermal conductivity, will constantly change as the solid nanoparticles gradually separate from the fluid. Unfortunately, these early studies on carbon nanotube-containing nanofluids have primarily focused on the enhancement of thermal conductivity, and very little experimental data is available regarding the stability of these nanoparticle suspensions.
Accordingly, there is a need for the development of a stable nanoparticle-containing fluid and methods for efficiently dispersing carbon nanoparticles into a desired heat transfer fluid to produce a nanofluid with a sustainable stability and consistent thermal properties. Hence, the present invention provides a nanofluid, which comprises a conventional heat transfer fluid, carbon nanoparticles, metal oxide nanoparticles and a surfactant. The metal oxide nanoparticles in combination with the surfactant are used to facilitate the dispersion of the carbon nanoparticles and to increase the stability of the nanofluid. The present invention also provides methods for preparing such carbon nanoparticle-containing fluid with enhanced thermal conductive properties.